The present invention relates to a cutting tool insert for machining ferrous materials, composed of a sintered ceramic material with a content of at least 70 vol.-% aluminum nitride, in which carbides, nitrides and borides of titanium, tungsten and niobium as hard substances, and metal oxide compounds as additional components, are uniformly distributed, and to the use of the cutting tool insert for machining a ferrous material with a carbon content of up to 1.2%.
Hard-metal materials have been used in the past for the machining of steels. The materials known as hard metals consist of mixtures have been used in the past for the machining of steels. The materials known as hard metals consist of mixtures of several metal carbides, mainly tungsten and titanium carbide, and usually cobalt as the binding metal.
Further developments of these materials led to the cutting tool compositions known from AT Pat. No. 266,465. In such compositions one or more nitrides from the group, titanium nitride, aluminum nitride, vanadium nitride, zirconium nitride, tantalum nitride and hafnium nitride are dispersed in an amount of 1 to 99 parts by volume per volume-part of a metal of the group, iron, cobalt, nickel and their alloys, the total composition containing also 1 to 95 vol.-% of a refractory aluminum compound, such as aluminum oxide and aluminum carbide (Al.sub.4 C.sub.3), with respect to the total volume of nonmetallic components.
In a like manner, U.S. Pat. No. 3,409,416 teaches the use of molybdenum, tungsten, rhenium and their alloys with one another and with chromium and their alloys with a small proportion of a metal selected from the group of aluminum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, manganese, iron, cobalt and nickel, instead of the binding metals mentioned in AT Pat. No. 266,465.
In the production of highly refractory materials and cutting tools, the common use of binding metals such as iron, cobalt and nickel or their alloys with nitrides of titanium, aluminum, niobium, vanadium, zirconium, tantalum and/or hafnium, and of the addition of a refractory aluminum compound, is described in DE-AS No. 12 95 855, in which up to 95% of the stated nitrides can be replaced by the nitrides of beryllium, boron, thorium or uranium, or borides of titanium, zirconium, cerium, tungsten, molybdenum or chromium, or by carbides of titanium, zirconium, tantalum or niobium, or by oxides of zirconium, magnesium or thorium.
The common disadvantage of these prior art techniques prescribing the use of binding metals is that the use of metal components lowers temperature stability. There is a danger of scaling under high temperature stress if molybdenum and tungsten are used as binding metals. Other common binding metals, especially nickel, iron and cobalt, have a relatively low softening temperature which if exceeded results in the plastic deformation of the cutting tool insert and the end of its useful life.
In U.S. Pat. No. 3,108,887, a material has also been proposed which has as its chief component aluminum nitride in the amount of more than 50%. The oxygen, boron, nitrogen, silicon and carbon compounds of aluminum, boron, silicon and rare earths and the transition metals such as titanium and zirconium are proposed as additives.
The examples given in that disclosure show compositions of 96 and 80 weight-percent of aluminum nitride, the balance being aluminum oxide and impurities. These compositions were proposed for the production of components for rocket engines, such as rocket nozzles, and for the treatment of molten metals. The hardness of this material is given as 7 to 8 on the Mohs scale, and as 1200 Knoop. The ultimate tensile strength at room temperature is 38,500 psi, corresponding to 265 MPa. The low hardness and low strength make it apparent that this material is still unsuitable for the production of cutting tool inserts, especially those which can serve for the machining of ferrous materials.
Cutter insert materials based on aluminum oxide with various additives, especially zirconium oxide, have also been proposed. For instance, the examples of DE-OS No. 27 41 295 provide for the addition of titanium carbide, titanium nitride, yttrium oxide and metals such as molybdenum and nickel, plus zirconium oxide, to a material based on aluminum oxide.
On the other hand, DE-OS No. 29 23 213 proposes compositions on the basis of aluminum oxide, zirconium oxide and magnesium oxide, but the zirconium oxide is not stabilized. Improvements of performance heretofore considered impossible have been achieved with cutting tool inserts made mostly on the basis of ceramic oxide materials. But such performance in many cases is still unsatisfactory, and there remains a constant demand for yet improved cutting tool inserts especially in the machining of ferrous materials of low carbon content, i.e., steels.
Since high cutting speeds are generally desired, at which great resistance to heat is required in the cutting tool insert, the improvement of high-temperature stability and especially resistance to thermal shock is highly desirable. Cutter inserts having improved thermal shock resistance are desired especially for short engagement time, interrupted cuts, and turning operations involving unequal depths of cut. Hard metals are still used predominantly for such applications since the known oxide ceramics do not have sufficient resistance to thermal shock for this purpose. At the same time allowance must be made for the disadvantage of lower cutting speeds, since the binding metals used have only very little high-temperature stability.
It is therefore an object of the present invention of overcoming the existing problems and of developing a cutting tool insert of improved resistance of heat, especially one having an improved resistance to thermal shock.